The invention relates to an optical waveguide which is structured in a core layer which is located on a buffer layer and is covered by a cladding layer, the buffer layer being applied to a substrate, an optical component which is constructed on a substrate and is provided at least partially with a cladding layer and to two processes for producing a waveguide of this type according.
The invention is based on a priority application DE 100 41 174.6 which is hereby incorporated by reference.
Optical glass waveguides which are used in optical components such as an arrayed waveguide grading (AWG), a directional coupler or a star coupler, are produced by structuring a core layer doped, for example, with boron, phosphorus or germanium. This core layer is applied to a buffer layer. The latter consists, for example, of silicate (SiO2) and is grown by oxidation under high-pressure steam on a silicon substrate (Si). This buffer layer serves to insulate the core layer from the silicon substrate which has a very high refractive index. The optical waveguides are structured, for example, by dry etching into the core layer and are then covered by a cladding layer several xcexcm thick and made of silicate glass doped with boron, phosphorus or germanium.
Planar optical waveguides of this type in silicate glass have many applications in optical components for telecommunications. Generally however, these glass layers and therefore the optical components produced therefrom are not birefringence-free. This leads to uncontrollable polarisation-dependent losses in optical systems which are unacceptable when perfect operation is required.
In the meantime it has become known that the birefringence in the optical waveguide, which causes the TE-wave (electrical transversal component of the electromagnetic wave) of the optical signal to spread at a different speed in the waveguide compared with the TM-wave (magnetic transversal component of the electromagnetic wave), can be attributed to the use of silicon as substrate. The various thermal coefficients of expansion of glass layer and substrate material generally lead in the high temperature processes of glass production to thermally induced stresses in the glass layer which lead to birefringence.
The use of a glass substrate (SiO2) instead of silicon allows the stress and therefore the birefringence to be reduced but it is still too high for practical applications (S. Suzuki, Y. Inoue and Y. Ohmori, Elect. Lett., Vol. 30, No. 8 (1994), pp. 642-643). A process is also known in which in a plurality of additional process steps grooves are subsequently etched into the finished optical component to compensate for the stresses (E. Wildermuth et al, Electronics Lett., Vol. 34, No. 17 (1998), pp. 1661-1662).
Here, however, a process is aspired to in which the birefringence is already compensated during production of the glass layers and waveguides without additional process steps. Based on the publications by S. Suzuki et al, Electronics Lett., Vol. 33, No. 13, pp. 1173-1174 and S. M. Ojha et al, Electr. Lett., 34(1), (1998), pp. 78, a process is described in the article by Kilian et al, J. Lightw. Technol. Vol. 18(2), (2000), pp. 193 for producing birefringence-free planar optical waveguides. The process is based on the use of flame hydrolysis deposition (FHD) to cover the waveguides with a cladding layer. In this case, the cladding layer consists of highly doped silicate glass SiO2. Boron and phosphorus, for example, are used as dopants to adjust the refractive index. The quantity of boron atoms used allows the thermal expansion of the cladding layer to be increased such that cladding layer and silicon substrate have approximately the same thermal coefficient of expansion. It could be shown that optical waveguides have birefringence-free properties when the thermal coefficients of expansion of the cladding layer and of the substrate are the same.
This result was calculated with the aid of stress and mode simulation and is shown in FIG. 1. FIG. 1 shows the effective refractive indices of the TE-mode and TM-mode and the resulting birefringence (difference in the mode indices nTExe2x88x92nTM) versus the thermal coefficient of expansion of the cladding layer. With a thermal coefficient of expansion of the cladding layer of 3.65xc3x9710xe2x88x926 Kxe2x88x921, which almost corresponds to the value of the thermal coefficient of expansion of the silicon substrate of 3.6xc3x9710xe2x88x926 Kxe2x88x921, the resulting birefringence is zero.
The quantity of boron atoms used as dopant in this case in order to achieve a birefringence-free waveguide leads to sensitivity to moisture in the doped cladding layer. As a result, optical modules which are provided with a cladding layer of this type are unstable with respect to moisture and this can even lead to destruction of the cladding layer (crystallising out) and therefore of the entire optical component. A solution to this is provided if an additional protective layer is applied to the cladding layer, but moisture can still attack the cladding layer at the edges of such optical modules.
The invention is based on the object of producing optical waveguides integrated in optical modules which have a birefringence which is as low as possible, this property of birefringence and the component""s stability to moisture enduring over a long period.
The object is achieved according to the invention by an optical waveguide which is structured in a core layer which is located on a buffer layer and is covered by a cladding layer, the buffer layer being applied to a substrate, wherein a strip-shaped waveguide base of thickness d is formed between buffer layer and optical waveguide, which waveguide base is completely covered laterally by the cladding layer and has the optical waveguide structured thereon, and the cladding layer consists of a vitreous material doped with foreign atoms to impart a birefringence-free property to the optical waveguide, an optical component which is constructed on a substrate and is provided at least partially with a cladding layer, wherein the optical component has an optical waveguide as described above and by a process for producing an optical waveguide in which a buffer layer is applied to a substrate, to which buffer layer a core layer is applied, the optical waveguide being structured into the core layer, wherein a strip-shaped waveguide base of thickness d is formed from the buffer layer below the optical waveguide, and in that subsequently both the optical waveguide and the portion of the waveguide base not covered by the waveguide is covered by a cladding layer and a process for producing an optical waveguide in which a first buffer layer is applied to a substrate, to which first buffer layer a core layer is applied, the optical waveguide being structured into the core layer, wherein a further buffer layer is applied to the first buffer layer before the core layer is applied, from which further buffer layer a strip-shaped waveguide base of thickness d is formed, and in that subsequently both the optical waveguide and the portion of the waveguide base not covered by the waveguide is covered by a cladding layer.
Application of an optical waveguide along a strip-shaped structured buffer layer, formed as a waveguide base, allows a reduction in the thermal coefficient of expansion of the cladding layer with simultaneous birefringence-compensated waveguide. Accordingly, the quantity of dopants (for example boron atoms) no longer has to be so large for the cladding layer. This has the enormous advantage that optical components which comprise optical waveguides of this type remain birefringence-free for a long period for optical signals transmitted in the optical waveguides, and the cladding layer is moisture resistant.
The invention minimises in a simple manner the negative effects produced by the difference between the coefficients of expansion of the substrate and the waveguide. In the ideal case it is sufficient for this purpose to provide a waveguide base of a certain thickness. A suitable cladding layer is also advantageously selected, for example in a doped cladding layer the doping is appropriately selected to optimise the minimisation. Silicon, quartz glass, ceramic or a polymer, for example, can be used as substrate. An optical material, an amorphous optical material, glass or a primer for example is used as cladding layer. An optical material, an amorphous optical material, glass or a polymer for example is used as waveguide. An optical material, an amorphous optical material, glass or a polymer for example is used as buffer layer. The waveguide base is formed in one configuration from the buffer layer already formed, for example by etching. The waveguide base is accordingly made of the same material as the buffer layer.